Carbon-carbon reinforced materials, first developed in connection with space related programs, offer advantages over other construction materials where high strength to weight ratios are required and where high temperatures will be part of the environment. Thus while most metal alloys demonstrate drastic reductions in tensile strength as temperatures rise, ultimately losing all practical usefulness at temperatures in the range of 1500.degree. to 2000.degree. F., carbon-carbon reinforced composites have been developed which maintain useful tensile strengths at temperatures up to 3500.degree. and higher when coated with oxidation resistant materials. While tensile strengths of up to 40,000 psi have been achieved heretofore using laminates constructed of plies of inter-woven resin impregnated carbon or graphite cloth densified by liquid impregnation techniques, higher loads can only be carried by using a relatively large number of plies, undesirably increasing the volume and weight of the part to be produced therefrom.
Generally, reinforced carbon-carbon composite substrates are constructed of carbon fibers bound by a carbon matrix. Carbonaceous fibers such as polyacrylonitrile, rayon, and pitch based fibers are utilized. The original fibers are converted to carbon or graphite through pyrolysis techniques and are then impregnated with carbonizable liquid materials. The impregnated fibers are available either in the form of interwoven cloth or unidirectional "tape" in which bundles of the fibers are laid parallel to one another in a single direction without any cross weave fibers interconnecting same. These impregnated carbon fiber materials are used as plies to form a laminate of the desired shape, weight, and volume through a process which cures the impregnated resin materials. The cured laminate is then subjected to pyrolysis to decompose the cured resin to carbon. The resulting product is normally quite porous and must be densified using either liquid impregnation techniques or chemical vapor deposition techniques. As noted above, the resulting reinforced carbon-carbon composite material can then be treated to form an oxidation resistant coating thereon which allows the final part to be utilized at relatively high temperatures without mass loss due to oxidation.
Because they are less porous and provide more reinforcing fibers per unit volume, resin impregnated carbon filament unidirectional tapes provide higher strengths along the direction of the fibers than do carbon-carbon woven cloth plies. However, the reduced porosity of the unidirectional tapes presents problems during cure and densification procedures as resin movement is hampered and the necessary expulsion of gaseous products from resin decomposition to carbon also becomes a problem. Further, the thermal coefficient of expansion of carbon filament unidirectional tapes is greater in the direction of width than length thus introducing disruptive forces during the formation of a composite laminate having non-aligned tape plies or a combination of tape and cloth plies. Thus there exists a need for products which can take advantage of the higher directional strengths achievable through use of unidirectional tapes and for processes for producing same in a manner which eliminates the problems imposed through use of these denser laminate plies.